Tritium and deuterium impregnated targets for neutron generators

ABSTRACT

In rare-earth neutron targets, there is a tendency for the hydrided rare-earth film to flake off the metal substrate because of the formation of the brittle tri-hydrides. The present invention provides targets which alleviate this difficulty. In these novel targets there is located between the substrate and the hydrided rare-earth film an intermediate film of a further metal, the further metal being selected to diffuse readily into the metal of the first-mentioned film under solid-state conditions and form a solid solution or compound therewith, and to adhere well to the substrate, and being sufficiently thin not to reduce substantially the absorption of hydrogen by the firstmentioned film by dilution of the first-mentioned film. Suitable intermediate films include nickel and gold.

United Mates Patent Stark 1 Aug. 8, 1972 [54] TRITIUM AND DEUTERIUM3,320,422 5/1967 St. John ..250/84.5

IMPREGNATED TARGETS FOR NEUTRON GENERATORS Primary Examiner.lames W.Lawrence Assistant Examiner-Davis L. Willis [72] Inventor:lgrgifigdSutherland Stark, Baldock, AttorneymLarson Taylor and Hinds[73] Assignee: National Research Development [57] ABSTRACT CorporatlonLondon England In rare-earth neutron targets, there is a tendency for[22] Fil d; Feb, 13, 1969 the hydrided rare-earth film to flake off themetal substrate because of the formation of the brittle tri- [211 Appl'799075 hydrides, The present invention provides targets which alleviatethis difficulty. In these novel targets there is [30] ForeignApplication Priority Data located between the substrate and the hydridedrareearth film an intermediate film of a further metal, the 1968 GreatBmam "8321/68 further metal being selected to diffuse readily into themetal of the first-mentioned film under solid-state [g2]..250/84.5(,;1;l13/g/10: conditions and form a Solid Solution orcompound gm/71 therewith, and to adhere well to the substrate, and 1 1e0 care being sufficiently thin not to reduce substantially theabsorption of hydrogen by the first-mentioned film by [56] ReferencesClted dilution of the first-mentioned film. Suitable inter- UNTTEDSTATES PATENTS mediate films include nickel and gold.

3,124,71 l 3/1964 Reifenschweiler....250/84.5 X 6 Claims, N0 DrawingsPATENIEMUB 8 m2 3.683.190

mvsmom DONALD S. STARK BY Jmoq J y/m 564/5 ATTOR N EYS TRITIUM AND DEUWMREGNATED TARGETS FOR NEUTRON GENERATORS BACKGROUND OF THE INVENTION thelanthanon elements are defined. In the aforesaid 1O specification, thefilm is deposited on a substrate metal with which it does not readilyalloy. The substrate is so much thicker than the film that alloyingwould partially or completely inhibit the absorption of hydrogen. Thechoice of substrate metal is thus limited; molybdenum, tungsten,tantalum and chromium are the most suitable.

For a sealed neutron generator, e.g. as described in United KingdomBritish specification No. 1,088,088, having a high neutron output andlong life, a comparatively thick film must be used to compensate forsputtering by incident deuterium or tritium ions accelerated on to thetarget. It is difficult in practice to obtain an incident ion beam ofuniform power density. This leads to a variation of temperature over thetarget area, and in preventing an excessive target temperature in thehightemperature regions of the target, it is difficult to prevent thecooler regions falling below about 200 C. At these lower temperaturesthe absorption of ions from the beam can lead to the formation of thetrihydride of the film metal, e.g., erbium trihydride; at highertemperatures only the dihydrides are formed. The trihydrides areextremely brittle compared with the dihydrides, and it is found thatalthough comparatively thin films of 0.00020.0005 cm can be made whichremain adherent, long-life films of 0.0025 cm and thicker disintegrateupon partial trihydriding, leaving clean bare areas of substrate.

It is an object of the present invention to provide a form of neutrontarget which reduces this tendency to disintegration.

SUMMARY OF THE INVENTION According to the present invention a neutrongenerator target comprises a metal film impregnated with hydrogenisotope, said metal being chosen from the group consisting of yttrium,scandium and the lanthanons, the metal film being suprmrted on a metalsubstrate, preferably a substrate which does not alloy readily with themetal of the film, wherein there is located between the substrate andthe film an intermediate film of a further metal, said further metalbeing selected to diifuse readily into the metal of the firstmentionedfilm under solid-state conditions and form a solid solution or compoundtherewith, and to adhere well to the substrate, and being sufficientlythin not to reduce substantially the absorption of hydrogen by thefirst-mentioned film by dilution of the first-mentioned film.

The formation of the solid solution or compound by intermetallicdiffusion in the solid state is to be distinguished from the formationof the alloy by melting. The further metal is preferably selected sothat the alloy formed between it and the metal of the first-mentionedfilm does not melt at the temperatures used when evaporating thefirst-mentioned film on to the intermediate film and subsequentlyloading the former film with hydrogen isotope. For example, availabledata indicates that erbium forms alloys with the following metals,listed in order of decreasing alloy melting points, and thereforedecreasing suitability: beryllium, gold, silver, nickel, cobalt, copper.Beryllium has the disadvantages of high toxicity. Other suitable metalsmay be used.

The thickness of the intermediate film is made much less than that ofthe hydrogen-absorbing film in order to limit the dilution of the latterby difiusion when the solid solution or compound is formed, but issufficient to increase its adhesion to the substrate.

The intermediate film is preferably evaporated on to the substrate usingan evaporation geometry similar to that subsequently used forevaporating the film of hydride-forming metal on to the intermediatefilm, in order to produce a more uniform thickness ratio of the twofilms over the target surface.

Because of the thinness of the intermediate film it is preferred,despite any buffer action which the latter may provide, to use as thesubstrate a metal which does not alloy readily with the metal of thefirst-mentioned film, as in the prior art.

The present invention also provides a method of producing targets asaforesaid, and a sealed neutron generator comprising a target asaforesaid.

A drawing is included to further describe the invention. The drawing isa diagrammatic elevation in section of the neutron target of theinvention wherein 6 represents the metal substrate; 4 represents theintermediate metal film of a further metal and 2 represents the metalfilm impregnated with hydrogen isotope.

EXAMPLES OF THE INVENTION The following are examples of neutron targetsand methods of producing them, embodying the present invention.

EXAMPLE 1 A 0.0001 cm thick film of nickel was vacuum evaporated on to amolybdenum substrate and a 0.005 cm thick film of erbium subsequentlyevaporated on to the nickel. The target was loaded to a hydrogen/erbiumatomic ratio of 2.8, with practically no loss of erbium film integrity.At this ratio, an erbium film of such thickness without the intermediatenickel film disintegrates to a powder, leaving the substrate bare.Ultrasonic cleaning in toluene followed by an adhesivetape strip-test(in which Scotch tape is applied to the loaded erbium film andsubsequently pulled off) removed only about 8 percent of the erbiumfilm.

The intermediate nickel film can also be applied to the substrate byelectroplating, but vacuum evaporation is preferred since the use ofsimilar processes for the nickel and erbium films produce a more uniformnickel/erbium thickness ratio. If this ratio is too high over any partof the target area, the excess of nickel appears to encourage theformation of a low meltingpoint nickel/erbium alloy; where such meltingis observed to occur, there is a greater tendency for the erbium layerto flake off when loaded with hydrogen. For this reason, and because thequantity of nickel to be evaporated is so much smaller than the quantityof erbium, the evaporation boat may be plated uniformly with the nickel,e. g., by electroplating, instead of loaded with wire or particles inthe usual manner (and as in Example l in order to obtain a betterdefined geometry similar to that for the erbium. This plated-boattechnique may be used for intermediate films of metals other thannickel. The film of erbium, or other hydrideforrning metal, ispreferably evaporated (as in Example 1 by the two-stage evaporationtechnique described in copending application Ser. No. 3 129/68.

EXAh/IPLE 2 A 0.0001 cm thick gold film was vacuum evaporated on to amolybdenum substrate and a 0.005 cm thick erbium film evaporated on tothe gold, using the same techniques as in Example 1. The erbium film wasloaded to a hydrogen/erbium atomic ratio of 2.8. There was no visibleflaking or crumbling of the erbium film, as would have been the casewithout the intermediate gold film. The erbium film withstood ultrasoniccleaning in toluene with the loss of only about very small pinholes(about 0.25 mm in diameter).

Although the above examples relate only to the use of erbium with amolybdenum substrate, the remaining Ianthanons, yttrium or scandium canbe used, and other substrates such as tungsten, tantalum or chromium, asuitable metal being selected for the intermediate film in each case,e.g., one of the six (beryllium, gold, silver, nickel, cobalt, copper)mentioned above.

I claim:

1. A neutron generator target comprising a metal fih-n impregnated withhydrogen isotope, said metal being chosen from the group consisting ofyttrium, scandium and the lanthanons, the metal film being supported ona metal substrate which does not alloy readily with the metal of thefilm, wherein there is located between the substrate and the film anintermediate film of a further metal, said further metal having athickness much less than that of said metal film impregnated withhydrogen isotope and being selected to diffuse readily into the metal ofthe first-mentioned film under solidstate conditions and form a solidsolution or compound therewith, and to adhere well to the substrate, andbeing sufiiciently thin not to reduce substantially the absorption ofhydrogen by the first-mentioned film by dilution of the first-mentionedfilm.

2. A target as claimed in claim 1 wherein the further metal is selectedfrom the group consisting of beryllium, gold, silver, nickel, cobalt andcopper.

3. A target as claimed in claim 2 wherein the firstmentioned film is oferbium and the further metal is selected from the group consisting ofnickel and gold.

4. A target as claimed in claim 1 wherein the intermediate film isapproximately one-fiftieth of the thickness of the first-mentioned film.

5. A target as claimed in claim 1 wherein the substrate is selected fromthe group consisting of tungsten, tantalum, chromium and molybdenum.

6. A target as claimed in claim 1 wherein said impregnated metal film ismore than 0.0025 cm thick.

2. A target as claimed in claim 1 wherein the further metal is selectedfrom the group consisting of beryllium, gold, silver, nickel, cobalt andcopper.
 3. A target as claimed in claim 2 wherein the first-mentionedfilm is of erbium and the further metal is selected from the groupconsisting of nickel and gold.
 4. A target as claimed in claim 1 whereinthe intermediate film is approximately one-fiftieth of the thickness ofthe first-mentioned film.
 5. A target as claimed in claim 1 wherein thesubstrate is selected from the group consisting of tungsten, tantalum,chromium and molybdenum.
 6. A target as claimed in claim 1 wherein saidimpregnated metal film is more than 0.0025 cm thick.